We've all come to accept that tattoos are permanent — no matter how hard we try to scrub, burn, or laser off the remnants of regrettable ink. But despite the tattooing practice being in existence for some 5,200 years, scientists have never been able to pinpoint the exact reason the pigment lasts so long on the skin. A new study published in the Journal of Experimental Medicine revealed that tattoos don't shed with the rest of your skin because the pigment is trapped deep in the dermis, thanks to a little-known immune system cell called a macrophage.